Field of the Invention
The present invention relates to an optical system suitable for a photographic camera, a digital still camera, a digital video camera, or the like.
Description of the Related Art
Recently, an image pickup optical system with a wide angle of view and a high quality (i.e., high resolving power and low distortion) has been required. In many cases, a wide-angle lens with a wide angle of view disposes a lens unit having a negative refractive power as a first unit. However, if the lens unit having the negative refractive power is disposed as the first unit, typically, negative distortion (barrel-shaped distortion) tends to occur. Japanese Patent Laid-open No. 2014-10286 discloses a zoom lens which uses an aspherical lens as a surface closest to an object side to suppress an absolute value of distortion while it is an extremely-wide angle zoom lens in which an angle of view at a wide-angle end is greater than or equal to 100 degrees.
However, even in a lens with little distortion, a captured image may look distorted due to the emphasis of perspective. In addition, appropriate distortion changes depending on an object and a composition of the captured image. Accordingly, the distortion needs to be variable.
Japanese Patent Laid-open No. H8-163426 discloses a method of performing an electric correction in an optical device (camera) to change the distortion. In the method of Japanese Patent Laid-open No. H8-163426, however, a current image is processed at the time of the correction, and accordingly the image is deteriorated. For example, when an image pickup optical system is attached to the camera having an optical finder, it is not possible to perform appropriate framing since the distortion correction cannot be confirmed through the optical finder.
Japanese Patent Laid-open No. 2008-134323 discloses an image pickup optical system which can optically vary the distortion to correct the emphasis of perspective while maintaining an image quality. According to the configuration of Japanese Patent Laid-open No. 2008-134323, a lens unit in the image pickup optical system can be moved in an optical axis direction to change the distortion.
In the configuration of Japanese Patent Laid-open No. 2008-134323, only a single lens unit in the image pickup optical system is moved, and accordingly an amount of change of the distortion is small. Furthermore, in the configuration of Japanese Patent Laid-open No. 2008-134323, the image quality is deteriorated if a sufficient amount of change of the distortion is to be obtained. Accordingly, it is difficult to vary the distortion while maintaining a good image quality.